1. Field of the Invention
The present invention relates to a pulse tube refrigerator, for instance, the pulse tube refrigerator applied to a cryopump and the like.
2. Description of the Related Art
A pulse tube refrigerator is a refrigerator with which low temperatures of less than 30 K (kelvin) can be obtained, and in recent years, it is desirable to have a refrigerator which has a refrigeration capacity of making cryogenic temperatures of less than 10 K, particularly, a 4 K level, in practice.
A pulse tube refrigerator, a basic type of refrigerator invented by Giford, is built up with only a regenerator and pulse tubes in addition to a compressor. Except for the basic type, the pulse tube refrigerator is improved in refrigeration efficiency by making a phase angle between pressure fluctuation in the pulse tube and displacement of a working gas (a gas column, gas piston) in a good condition, through providing various phase control mechanisms in the high temperature side of the pulse tube.
The pulse tube refrigerator includes such phase control mechanisms as an orifice type in which a buffer (reservoir tank) is connected to the high temperature side of the pulse tube through an orifice, a double inlet type in which a by-pass valve which connects the high temperature side of the pulse tube with the high temperature side of the regenerator, is added to the orifice type, and a four-valve type in which the high pressure side and the low pressure side of the compressor is connected to the high temperature side of the pulse type also, and so forth.
However, since the orifice type and the double inlet type have the buffer, there arises a disadvantage that the refrigerator becomes big. On the other hand, the four valve type has also a disadvantage that it is not easy to obtain a high refrigeration efficiency in the cryogenic temperature area of below 10 K, though it can be downsized.
Here, aside from the disadvantages in the pulse tube refrigerator, improvement has been made for a regenerator used in the refrigerator to boost the refrigeration efficiency in the cryogenic temperature area below 10 K.
That is, hitherto, copper or copper alloy has been used as a regenerator in the refrigerator which generates higher than 30 K in the cryogenic temperature area, and lead is used in the refrigerator for the use in the temperature of 10 K and 30 K. This is because each metal has sufficient specific heat in the temperature areas of each refrigerator so that a sufficient regenerative capacity is produced.
However, copper, copper alloy, and lead is the regenerator, have the property that each specific heat becomes low, in case of copper or copper alloy at below 30 K, and in case of lead at below 10 K. Therefore, in the lower temperature areas, no matter how much energy in applied to the refrigerator, sufficient regeneration can not be performed and consequently the cryogenic temperature can not be obtained.
For such disadvantages, the improvement in the regenerative material by using specific magnetic materials has been tried. That is, in the Japanese Patent Publication No Hei 7-92286, the Japanese Patent Publication No Hei 7-101134, U.S. Pat. No. 5,186,765 and U.S. Pat. No. 5,449,416, it is disclosed that magnetic material composed of a rare-earth element and a transition metal is used as a regenerator which can maintain a large specific heat even in the cryogenic temperature below 10 K. As the rare-earth metals, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb are cited, and as the transition metals, Ni, Co, and Cu are cited in each bulletin.
It should be mentioned that each bulletin shows that a refrigerator using these regenerative material generates the cryogenic temperature below 10 K, the refrigerator written in the bulletins has a movable portion in a low temperature (an expander or displacer) and is different from the pulse tube refrigerator relating to the present invention.
As above, since the magnetic regenerative material made of the rare-earth element and the transition metal show an excellent property at low temperatures below 10 K, it is conceivable that the application of the regenerative material to the regenerator of the four valve type pulse tube refrigerator also is effective for the improvement of the refrigeration efficiency and the achievement of the practical use level at low temperatures below 10 K.
However, when the magnetic regenerative material is combined with the four valve type pulse tube refrigerator, the improvement of the refrigeration efficiency will not be obtained in practice. The reason is that sufficient refrigeration efficiency may not be obtained through the four valve type pulse tube refrigerator itself.
That is, in four valve type, the working gas can come and go between the high temperature side of the pulse tube and the compressor when connecting the compressor with the high temperature side of the pulse tube through the orifice, valve, and the like. Therefore an excess load is exerted upon the compressor, resulting in the waste of refrigeration energy, which makes it difficult to enhance the refrigeration efficiency.
If the refrigeration efficiency is low, even once the cryogenic temperature below 10 K is created, the refrigeration capacity thereof is as little as at the milliwatt level. Then, there arises a disadvantage that if the input energy (power consumption) is not enough, the cryogenic temperature below 10 K can not be obtained.
In other words, when the magnetic regenerator is combined with the four valve type pulse tube refrigerator, with a small power consumption, the refrigerator can not continue to drive to the cryogenic temperature in which the specific property of the regenerative material is exploited.
It is an object of the present invention to provide a pulse tube refrigerator which generates cryogenic temperatures of below 10 K with low power consumption and with reduced size.